File BULK analysis

On my main drive, I have a 120gb hd.. then a 200gb backup. Now, I have less than 12gb free on my main drive, but I CANT FIND ANY OF IT. I know I have a lot of stuff on it, but I need a program to go through each folder, and give me the size of all folder, and were the BULK of the space is being taken up.

So does anyone have a program.. or make a program that can do that? It would be greatly appreciated.

Jeremy